/**
 * 21. 合并两个有序链表
 */
public class Solution_21 {
    /**
     * 方法二：递归
     * 
     * @param l1
     * @param l2
     * @return
     */
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // 递归出口
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        // 递归
        if (l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }

    /**
     * 方法一：迭代
     * 
     * @param l1
     * @param l2
     * @return
     */
    public ListNode mergeTwoLists1(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        }
        if (l2 == null) {
            return l1;
        }

        ListNode dummyHead = new ListNode(0);
        ListNode cur = dummyHead;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
            cur = cur.next;
        }
        if (l1 == null) {
            cur.next = l2;
        }
        if (l2 == null) {
            cur.next = l1;
        }
        return dummyHead.next;
    }
}
